Method and apparatus for controlling multiple suspension cranes

ABSTRACT

Four crane units support a load which it is desired to maintain in a horizontal position. The suspension point nearest the center of gravity is selected. An electric take-up motor controls the tension of the warp connected to each suspension point. The component load on the selected suspension point is varied in a range between upper and lower limit and this mid-point of the range is determined to provide a reference warp tension signal for controlling the speeds of the motors controlling the warps attached to the suspension points.

United States Patent [1 1 Niwa [75] Inventor:

[73] Assignee: Fuji Electric Co., Ltd.,

Kawasaki-shi, Kanagawa, Japan [22] Filed: Mar. 28, 1974 [21] Appl. No.1 455,828

Koichi Niwa, Kawasaki, Japan [30] Foreign Application Priority Data Mar. 28, 1973 Japan 48-35401 [52] U.S. Cl. .i 318/7; 318/71; 318/85; 318/98; 318/99 [51] Int. Cl. 1102p 7/68 [58] Field of Search 318/7, 6, 45, 71, 85, 77, 318/98, 99

[56] References Cited UNITED STATES PATENTS 3,452,261 6/1969 Tagliasacchi 318/77 III B [4 1 May 27, 1975 3,551,775 12/1970 Safluddin 318/71 1688167 8/1972 lvey 318/99 X 3,789,280 1/1974 Oldfield .i 318/77 X Primary Examiner-B. Dobeck Attorney, Agent, or Firm-Sughrue, Rothwell. Mion. Zinn & Macpeak [5 7] ABSTRACT Four crane units support a load which it is desired to maintain in a horizontal position The suspension point nearest the center of gravity is selected. An electric take-up motor controls the tension of the warp connected to each suspension point. The component load on the selected suspension point is varied in a range between upper and lower limit and this midpoint of the range is determined to provide a reference warp tension signal for controlling the speeds of the motors controlling the warps attached to the suspension points.

3 Claims, 8 Drawing Figures TENSlON I commune us 2 n SPEED CURRENT 0 REGULATOR REGULATOR PATENTEBIAAYZ? 191s EBBSAN, SHEET 1 FIGI H FIGZ A PIEPZ B CP4 F P3 9 I MAX CALCULATION MEMORY {MN W F cmcun IRCUIT 2 1 fib'/i2r& HG 3 CURRENT REGULATOR PATENTEU MAY 27 ms SHEET FIG. 5

FIG 6 FIG. 8

METHOD AND APPARATUS FOR CONTROLLING MULTIPLE SUSPENSION CRANES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of controlling operation of multiple suspension cranes adapted for lifting substantially large loads, up to thousands of tons.

2. Prior Art In use of a multiple suspension crane of this type, it has been a common practice to individually control the torques of electric motors carried by the respective crane units contained in the multiple suspension crane by the crane operator who is observing the warp tension meters associated with the respective crane units for control of the individual crane motors to avoid an excessively large amount of component load on a particular crane unit.

However, the center of gravity of the total load does not always coincide with the median point of all the suspension points, such a prior art system thus putting the burden of providing properly distributed component loads on the individual crane units upon the skill of the crane operator. As the amount of total load increases, a system operated in this manner becomes dangerous.

SUMMARY OF THE INVENTION A primary object of the present invention is to provide a method of controlling a multiple suspension crane containing four independently controllable crane units or a multiple thereof, which method assures safety in operation of the multiple suspension crane by calculating a reference warp tension level in a substantially practical and reliable manner without relying upon the skill of the operator and by controlling the warp tensions imposed on the individual crame units so as to maintain these tensions within a permissible range.

In accordance with the invention, there is provided a method for controlling multiple suspension cranes of the type which includes a set of four independently controllable crane units or an equivalent set of such crane units, each of the crane units including both an electric motor for taking up the warp of the crane unit and also a speed controller for controlling the speed of the electric motor, said method comprising the steps of determining a permissible range of component load imposed on the suspension point nearest the center of gravity of the total load by actually increasing and decreasing the amount of component load on said suspen sion point, and using the medium value of the permissible range of component load as a reference warp tension signal to compensate the speed control of the elec tric motor operating to take up the warp connected to said specific suspension point.

The method of the present invention is particularly applicable to four-point suspension cranes or eightpoint suspension cranes for controlling the operation thereof. The specific suspension point that is located nearest the center of gravity of the total load is known by comparing the sum of component loads on a pair of spaced suspension points along one side with that on another pair of spaced suspension points along the opposite side.

The permissible range of component load on the specific suspension point nearest the center of gravity of the total load is determined by actually increasing and decreasing the amount of component load imposed on the suspension point. If the component load on any of the remaining suspension points decreases down to zero or increases to its materially allowed limit during decrease of the load component on the specific suspension point, it indicates that the amount of load compo nent on the specific suspension point providing such result is the lower limit of the permissible range. Then. the component load on the specific suspension point is increased until the load component on any other sus pension point decreases down to zero or the compo nent load on the specific suspension point itself reaches its materially allowed limit, and the upper limit of the permissible load range on the specific suspension point is thus obtained.

Since the permissible range of component load on the specific suspension point nearest the center ofgravity of the total weight is determined in connection with the materially allowed limit on the other suspension points in the manner described. a reference signal corresponding to the medium value of the permissible load range on the specific suspension point will reliably control the amount of warp tension imposed on each of the crane units to a level substantially lower than its materially allowed limit.

The present invention will now be more specifically described in reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic perspective view of a loading weight to be suspended at four suspension points by the method of the present invention;

FIG. 2 is a plan view of same;

FIG. 3 is a diagrammatic view explaining the process of detecting the permissible load range on a specific suspension point and calculating the medium value thereof;

FIG. 4 is a block diagram illustrating the control cir cuit used in the practice ofthe method of the invention;

FIGS. 5 and 6 illustrate characteristics of operation of the control circuit; and

FIG. 7 is a schematic diagram showing the arrangement of suspension points in an eight-point suspension system.

FIG. 8 is a schematic diagram showing a portion of FIG. 4 in more detail.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I. a solid substance with total loading weight W is lifted and suspended at a plurality, for example four, ofsuspension points A to D. Usually, the center of gravity G of the total loading weight is offset from the center of the suspension points A to D and, therefore, with the loading weight lifted and kept in a horizontal position the component loads WA to WD imposed on the individual suspension points are not identical, and the specific suspension point nearest the center of gravity of the total load is subjected to a greater amount ofloading stress than any other suspension points,

FIG. 2 illustrates a plan view showing the relative positioning of the suspension points A to D and the center of gravity of the loading weight of FIG. I, in which the suspension points A to D form at least the four corners of a parallelogram.

Assuming that the resultant of the component loads WA and WE on the suspension points A and B is at the point Pl, the resultant of the component loads WC and WD on the suspension points C and D should be at the point P3 which is the intersection of the extended line. drawn from the point P] on the side AB through the center of gravity G of the total load, with the opposite side CD of the parallelogram; otherwise the load cannot assume the desired horizontal position.

Under such a condition, the moment balances are whim warm m WCPBC I WI).P3

With a given location of the center of gravity G, the resultant loads WA+WBl and WCd-WD) on the opposite sides AB and CD, respectively, can be calculated from Eq. (3). Accordingly, when the amount of component load WA on the point A is selected, the component load WB on the point B may be calculated. Also the location of the point P1 may be determined from Eq. (I), and thus the location of the point P3 may be obtained which determines the amounts of component loads WC and WD in accordance with Eq. (2),

Therefore. it should be understood that, when the value of one ofthe component loads on the four points suspending a loading wieght having a center of graavity G is selected and the location of the center of gravity G is known, then the component loads on the remaining three suspension points all can be calculated.

In accordance with the present invention, a warp tension control is provided by selecting the value or amount of the component load on the suspension point nearest the center of gravity of the total load such that there are no excessive component loads on any of the other suspension points.

Further in reference to FIG. 2, assuming that the suspension point A is nearest to the center of gravity G, the resultant loads (WA-i-WB) and (WC-HVD) on the opposite sides AB and CD, respectively, may be in the following relation WA WB WC WD (5) and a similar relation may be found with respect to the remaining opposite sides AC and BD. WA WC WB W0 is The resultant point P1 moves with change in the amount of the component load WA on the suspension point A nearest the center of gravity of the total load. When the point P1 moves, for example, to a new point P2, the point P3 also moves to another point P4 on the opposite side DC.

From Eqs. l (2). it is understood that as a resultant point between two spaced suspension points approaches either of the suspension points, the component load on the corresponding suspension point becomes greater, whereas the component load on the other suspension point decreases. ultimately down to zero when the resultant point reaches the first mentioned suspension point.

Under such a situation, the arrangement is essentially that of a triple suspension system and a further shifting of the resultant point would lose the possibility of the desired horizontal attitude of the load weight.

The component load on the specific suspension point A is increased so that the resultant point P1 approaches the suspension point A until the component load WA on the suspension point A reaches the materially allowed limit of warp tension thereon or the component load on either of the suspension points B or C diminishes to zero while the load on the specific suspension point A still remains the materially allowed tension limit.

At this final point, the amount of component load measured on the suspension point A is defined as the upper limit of the permissible range.

The component load WA is then reduced so that the point P1 moves away from the suspension point A and, during this process, the component load on the suspension point C will reach its materially allowed limit of warp tension or else the component load on the suspen sion point D decreases down to zero. Such a situation provides a lower limit on the permissible range of the component load expected on the suspension point A.

In this manner, the upper and lower limits on the component load permissible on the suspension point A are obtained, and the mid-point or medium value of the range between the upper and lower limits is employed as as set or reference tension value or signal for con trolling the warp tension on the suspension point A.

Continuous tension control on the specific warp which might be subject to the most severe operating condition assures safety and reliability in the operation of the multiple suspension crane.

When DC. motors are used for the warp take-up motors, the measurement of the warp tensions imposed on the individual crane units, that is, the component loads on the individual suspension points, is accomplished by detecting and measuring the magnitude of electric current passing through the armature of the warp take-up motor associated with each one of the crane units.

As shown in FIG. 3, the reference tension value I can be obtained by introducing the individual armature currents IA to ID from the warp take-up motors MA to MD to a limit memory circuit 3, whereby the maximum and minimum permissible values [max and lmin of the current IA within the motor MA are measured and memorized by gradually increasing and decreasing the current IA until at least one of the currents IA to ID reaches its allowed limit or diminishes down to zero and the medium value of the maximum and minimum currents is calculated in a calculation circuit 4, and the reference current value I is thus obtained.

Controllable rectifiers are designated at 2 in FIG. 3.

FIG. 4 is a schematic block diagram illustrating the control system for each of the motors M individually taking up warps 5 connected to the respective suspen sion point. Each warp 5 is taken up on a drum 6 rotated by the motor M. Although only one motor and control system combination is illustrated in the figure, it should be understood that similar take-up mechanisms are provided for the remaining warps 5.

As illustrated in the drawing, a speed reference input n,,, which is common to all take-up motors, is introduced to a comparison circuit 7 in which it is compared with the actual speed value detected by the tachometer generator 8, and the deviation obtained in the comparison circuit is amplified in the speed regulator 9. The amplified deviation is further compared in a comparison circuit 10 with the actual current value I obtained by the current detector 12. and the current deviation thus measured is introduced through a current regulator 11 to the controllable rectifier 2 for controlling the output thereof. FIG. 8 shows that the controllable rectitier 2 of FIG. 4 consists of a plurality of SCR"s which are gated by the output of the current regulator 11 for controlling the power applied to the motor M from a three phase A.C. source.

In this manner, a current with a magnitude related to the speed deviation is supplied to the motor M, providing the desired speed control to keep the motor speed at the reference speed value.

The particular embodiment illustrated in the draw ings includes a comparison circuit 13, a tension controller l4 and a switch 15 in each control circuit for using the reference tension value I in relation to the suspension point nearest the center of gravity of the total load.

By closing the switch 15 in the control circuit of the warp take-up motor for the suspension point nearest the center of gravity of the total load, the reference ten sion value 1 is fed to the comparison circuit 13 for comparison with the armature current I corresponding to the actual tension value, and the deviation AI thus obtained is introduced to the tension controller 14.

FIG. illustrates the input-output characteristic of the tension controller 14 by solid lines 51, wherein the output H of the tension controller exhibits a blind zone when the tension deviation AI remains within the range iAI, and a sharply decreasing property out of the blind zone. With the switch [5 closed, the output H is introduced to the comparison circuit 7 and added to the reference speed value n The combined value is one which can be obtained by upwardly shifting the output characteristic 51 of the tension controller 14 by n as indicated by dotted lines in FIG. 5. This can be illustrated as a speed n-vs.- tension l graph, as shown in FIG. 6, and which comprises a flat section 61 extending over the range of-LAI on each side of the reference tension value L and sharply decreasing sections 62, 63 blending to the opposite ends of the flat section.

Thus, the specific motor incorporating the control circuit with the closed switch 15 is controlled to provide the speed vs. tension characteristic of FIG. 6 so that, when the warp tension remains within the range of the reference tension value (I tAI the motor speed is retained at the reference speed value n whereas the motor speed substantially increases as the warp tension becomes lower than (I -Al and the speed substantially decreases as the warp tension becomes greater than (I t-A1,).

By giving the above speed-tension characteristic to the warp take-up motor MA for the suspension point nearest the center of gravity of the total load while providing the usual speed control related to the reference speed value 11,, to the other motors MB to MD for the remaining suspension points, the take-up tension on the motor MA can be retained at all times within the range (IMLAL), because, when the take-up tension on the motor MA is out of the permissible range (I ,At" \n for example. above the upper limit, the takeup speed will be automatically reduced in accordance with the characteristic of FIG. 6. thus reducing the amount of component load and hence warp tension on the specific suspension point back to within the range (I -L31 and similarly any reduction of takeup tension of the motor MA to less than (I AI will be automatically compen sated for to keep the take-up tension of the motor MA within the range (1pm.

Upon setting the amount of component load on one suspension point, the component loads on the remaining three suspension points are determined as previously described.

The width of the blind zone iAI is selected from the standpoint of stability desired in the control of the machine operation. A smaller width is desirable in order to reduce the amount of error in relation to the reference value of component load.

It will readily be apparent that that the warp tension on the suspension point nearest the center of gravity of the total load can be controlled and retained within a safe value range which automatically keeps the amount of warp tensions on the remaining suspension points less than their materially allowed value.

While the above description is made in particular reference to a four-point suspension system, it should be understood that the invention is also applicable to an eight-point suspension system in which the suspension point A], A2, B1, B2 D1, D2 are all on the opposite sides of a parallelogram, as shown in FIG. 7.

The eightpoint suspension system can be considered as a four-point system by setting identical component loads on one pair of suspension points A] and A2 and on another pair of suspension points B1 and B2 and so on. Under such a situation, the middle point A0 and B0 between the points A1, A2 or B1, B2 can be used as an equivalent suspension point to the spaced paired suspension points. In this manner, a multiple suspension system including a multiple of four suspension points can be considered as a four-point system by grouping the number of suspension points into four sets of adjacent suspension points.

What is claimed is:

1. A method for controlling a multiple suspension crane system of the type which includes a set of four independently controllable crane units or an equivalent set of such crane units for suspending a load by warps connected to four different suspension points on the load, each of the crane units including an electric motor operable to take up the warp of its crane unit and a speed controller for controlling the speed of the electric motor. said method comprising the steps of:

a. determining a permissible range of component load imposed on the specific suspension point near est the center of gravity of the total load by actually increasing and decreasing the amount of component load on said specific suspension point, and

b. using the mid-value of said permissible range of component load to produce a reference warp tension signal to compensate the speed controller of the specific electric motor operating to take up the warp connected to said specific suspension point.

2. An apparatus for controlling a multiple suspension crane system of the type which includes a set of four independently controllable crane units or an equivalent set of such crane units for suspending a load by warps connected to four different suspension points on the load, each of the crane units including an electric 7 motor operable to take up the warp of its crane unit and a speed controller for controlling the speed of the electric motor, said apparatus comprising;

a, means for determining a permissible range of component load imposed on the specific suspension point nearest the center of gravity of the total load by actually increasing and decreasing the amount of component load on said suspension point and b. means responsive to the midwalue of said pcrrnissible range for producing a reference warp tension signal to compensate the speed controller of the specific electric motor operating to take up the warp connected to said specific suspension point.

3. Apparatus as defined in claim 2. wherein said electric motor is a DC, motor herein said permissible range determining means comprises memory means for storing maximum and minimum permissible values of the armature current of said motor as said component load on said specific suspension point is increased and decreased and wherein said producing means com prises calculator means for calculating from said stored values said midw'alue and producing said reference ten sion signal therefrom each speed controller maintaining at a reference value the take-up speed of its associated motor; said apparatus further comprising tension controlling means responsive to the difference between the reference tension signal and the armature current of said specific motor for modifying the take-up speed of said specific motor to maintain within a preset range the tension on the warp connected to said specific suspension point thereby automatically maintaining the component load on all the suspension points below the upper limit of said permissible range. 

1. A method for controlling a multiple suspension crane system of the type which includes a set of four independently controllable crane units or an equivalent set of such crane units for suspending a load by warps connected to four different suspension points on the load, each of the crane units including an electric motor operable to take up the warp of its crane unit and a speed controller for controlling the speed of the electric motor, said method comprising the steps of: a. determining a permissible range of component load imposed on the specific suspension point nearest the center of gravity of the total load by actually increasing and decreasing the amount of component load on said specific suspension point, and b. using the mid-value of said permissible range of component load to produce a reference warp tension signal to compensate the speed controller of the specific electric motor operating to take up the warp connected to said specific suspension point.
 2. An apparatus for controlling a multiple suspension crane system of the type which includes a set of four independently controllable crane units or an equivalent set of such crane units for suspending a load by warps connected to four different suspension points on the load, each of the crane units including an electric motor operable to take up the warp of its crane unit and a speed controller for controlling the speed of the electric motor, said apparatus comprising: a. means for determining a permissible range of component load imposed on the specific suspension point nearest the center of gravity of the total load by actually increasing and decreasing the amount of component load on said suspension point, and b. means responsive to the mid-value of said permissible range for producing a reference warp tension signal to compensate the speed controller of the specific electric motor operating to take up the warp connected to said specific suspension point.
 3. Apparatus as defined in claim 2, wherein said electric motor is a D.C. motor, wherein said permissible range determining means comprises memory means for storing maximum and minimum permissible values of the armature current of said motor as said component load on said specific suspension point is increased and decreased, and wherein said producing means comprises calculator means for calculating from said stored values said mid-value and producing said reference tension signal therefrom, each speed controller maintaining at a reference value the take-up speed of its associated motor; said apparatus further comprising tension controlling means responsive to the difference between the reference tension signal and the armature current of said specific motor for modifying the take-up speed of said specific motor to maintain within a preset range the tension on the warp connected to said specific suspension point, thereby automatically maintaining the component load on all the suspension points below the upper limit of said permissible range. 